High hole mobility p-channel Ge transistor structure on Si substrate

ABSTRACT

The present disclosure provides an apparatus and method for implementing a high hole mobility p-channel Germanium (“Ge”) transistor structure on a Silicon (“Si”) substrate. One exemplary apparatus may include a buffer layer including a GaAs nucleation layer, a first GaAs buffer layer, and a second GaAs buffer layer. The exemplary apparatus may further include a bottom barrier on the second GaAs buffer layer and having a band gap greater than 1.1 eV, a Ge active channel layer on the bottom barrier and having a valence band offset relative to the bottom barrier that is greater than 0.3 eV, and an AlAs top barrier on the Ge active channel layer wherein the AlAs top barrier has a band gap greater than 1.1 eV. Of course, many alternatives, variations and modifications are possible without departing from this embodiment.

FIELD

The present disclosure describes a high hole mobility p-channel Getransistor structure on Si substrate.

BACKGROUND

Most modern electronic devices, e.g., computers and cellular telephones,may include semiconductor devices. Semiconductor devices may bemanufactured as discrete devices, e.g., transistors, and/or asintegrated circuits that may include many interconnected devices on asingle semiconductor substrate. The behavior of semiconductor devicesmay be manipulated by the controlled addition of impurities, e.g.,dopants. Design considerations may include device speed and powerconsumption when designing semiconductor devices and the electronicdevices that may include them.

For example, Silicon (“Si”) may be used as a substrate and Germanium(“Ge”) may be used for an active channel layer. The unequal latticeconstants of Si and Ge may require the inclusion of a transition orbuffer layer or layers between the Si substrate and the Ge activechannel layer. Without these buffer layers, lattice mismatch may resultin defects that may make a device inoperable or may cause a device tofail prematurely. In order to address lattice mismatch, a combination ofSi and Ge, e.g., Si_(1−x)Ge_(x) (x=0.4-0.7), may be used for thesebuffer layers. Although these buffer layers may address the latticemismatch, they may not provide a complete solution. A Ge active channellayer may suffer from parallel conduction between the active channel andthe Si_(1−x)Ge_(x) buffer layers because of Si_(1−x)Ge_(x)'s relativelylow band gap. As a result of parallel conduction between the activechannel and the Si1-xGex buffer layer, a relatively large gate voltagemay be needed to shut off a device. The Si_(1−x)Ge_(x)—Ge interface mayalso provide a relatively low valence band offset that may result ininadequate carrier confinement and an associated decrease in carriermobility. As a result, a semiconductor device constructed with a Sisubstrate, Ge active channel layer and Si_(1−x)Ge_(x) buffer layers, maybe slower and may consume more power than a semiconductor device withoutthese limitations.

BRIEF DESCRIPTION OF DRAWINGS

Features and advantages of the claimed subject matter will be apparentfrom the following detailed description of embodiments consistenttherewith, which description should be considered with reference to theaccompanying drawings, wherein:

FIGS. 1A and 1B illustrate two exemplary embodiments consistent with thepresent disclosure;

FIG. 2 depicts band diagrams versus lattice parameters for a pluralityof semiconductors;

FIG. 3 depicts an exemplary schematic representation of single domainGaAs on a Si substrate;

FIG. 4 depicts an exemplary TEM image of a 0.8 μm GaAs layer grown onSi; and

FIGS. 5A and 5B depict band offsets for two exemplary materialinterfaces.

Although the following Detailed Description will proceed with referencebeing made to illustrative embodiments, many alternatives,modifications, and variations thereof will be apparent to those skilledin the art.

DETAILED DESCRIPTION

Generally, this disclosure describes a method and apparatus forimplementing a high hole mobility p-channel Germanium (“Ge”) transistorstructure on a Silicon (“Si”) substrate. Hole mobility may affect theswitching speed of a device. Higher hole mobility may correspond to ahigher switching speed and may thereby provide faster deviceperformance. Ge may have a higher hole mobility relative to Si and toIII-V based compound semiconductors. Si may be a relatively more commonsubstrate than Ge that may be used for semiconductor fabrication. Si maybe relatively less expensive and may be available in relatively largediameter (e.g., 300 mm or more) ingots and wafers. State-of-the-art 65nm Si CMOS fabrication capabilities may be readily available. Inaddition, a high hole mobility p-channel Ge quantum well may beintegrated with an n-channel quantum well on a Si substrate. Forexample, Indium Gallium Arsenide (InGaAs), Indium Antimonide (InSb) orIndium Arsenide (InAs) may be used for the n-channel quantum well. Theintegrated p-channel and n-channel devices may be useful for ultra-highspeed low power CMOS logic applications.

Consistent with the present disclosure the apparatus may include abuffer and/or barrier layer or layers that may bridge material mismatchthat may be present between a Ge active device channel layer and the Sisubstrate. For example, the buffer and/or barrier layer or layers maybridge lattice constant differences that may be present between the Sisubstrate and the Ge active device channel layer. The buffer and/orbarrier layer or layers may further provide hole confinement within theGe active device channel layer. The buffer and/or barrier layer materialmay also reduce or eliminate parallel conduction between the Ge activedevice channel layer and the buffer and/or barrier layer.

FIGS. 1A and 1B illustrate two exemplary embodiments of the presentdisclosure. FIG. 1A depicts a representation of a layer structure of asemiconductor device 100 in accordance with one exemplary embodiment ofthe present disclosure. A Si substrate 110 may be provided. The Sisubstrate 110 may be p-type or n-type. The Si substrate 110 may have aresistivity in the range of about 1 Ω-cm to about 50 kΩ-cm, includingall values and increments therein. As used herein, “about” may beunderstood to mean within ±10%, for example, the Si substrate 110 mayhave a resistivity in the range of 1±0.1 Ω-cm to 50±5 kΩ-cm. The Sisubstrate 110 may further include a (100) offcut in the range of about2° to about 8°, including all values and increments therein, toward the[110] direction. In other words, the Si substrate 110 may be cut fromthe top surface (100) of an ingot but at an angle relative to thesurface of the ingot.

A GaAs (Gallium Arsenide) nucleation layer 120 may then be grown on theSi substrate 110. The GaAs nucleation layer 120 may be relatively thinwith a thickness in the range of about 30 Å (Angstrom) to about 500 Å,including all values and increments therein. The GaAs nucleation layer120 may be grown on the Si substrate 110 at relatively low temperatures,i.e., temperatures in the range of about 400° C. to about 500° C.,including all values and increments therein. The GaAs nucleation layer120 may be formed via metal organic chemical vapor deposition (MOCVD) ormolecular beam epitaxy (MBE), or another such process. The GaAsnucleation layer 120 may fill the lowest Si substrate 110 terraces withatomic bi-layers of GaAs material. The GaAs nucleation layer 120 maycreate an anti-phase domain-free “virtual polar” substrate.

A first GaAs buffer layer 130 may then be grown on the GaAs nucleationlayer 120. The first GaAs buffer layer may have a thickness in the rangeof about 0.2 μm to about 1.0 μm, including all values and incrementstherein. The first GaAs buffer layer may be grown at temperatures in therange of about 400° C. to about 600° C., including all values andincrements therein. Growth of the first GaAs buffer layer 130 mayinclude thermal cycle annealing. The thermal cycle annealing may reducedislocations that may be present in the crystal structure of the firstGaAs buffer layer 130 and/or the GaAs nucleation layer 120 at or nearthe interface with the Si substrate 110. Dislocations may be caused bylattice mismatch between GaAs and Si.

A second GaAs buffer layer 140 may then be grown on the first GaAsbuffer layer 130. The second GaAs buffer layer 140 may have a thicknessin the range of about 0.2 μm to about 5.0 μm, including all values andincrements therein. The second GaAs buffer layer 140 may be grown atrelatively higher temperatures, i.e., temperatures in the range of about500° C. to about 650° C., including all values and increments therein.Growth of the second GaAs buffer layer 140 at the relatively highertemperatures may provide relatively higher structural quality of thelayer 140.

A doped layer 145 may then be provided. The doped layer 145 may provideholes (charge carriers) to a Ge active channel layer, e.g., layer 160.The doped layer 145 may be grown on the second GaAs buffer layer and maybe a relatively thin (i.e., thickness less than 50 Å) layer of dopedGaAs or a delta-doped As (Arsenic) layer. The dopant may be Beryllium orCarbon, for example, and may provide holes, i.e. acceptors. Growth of adoped layer, prior to forming an active device channel, e.g., quantumwell, may be considered an inverted doping structure. In anotherexemplary embodiment, growth of a doped layer may not occur until afterthe growth of an active device channel, for example, on a top barrier.Growth of a doped layer after growth of an active device channel may beconsidered a normal doping structure. The doping may be δ-doping,modulation doping, flat doping or another type of doping. δ-doping maybe understood to yield dopant atoms that may be spatially confinedwithin one atomic layer, i.e., a delta-function-like doping profile.Modulation doping may be understood to yield a nonuniform,quasi-periodic distribution of dopant atoms across a doped layer. Flatdoping may be understood to yield a substantially uniform distributionof dopant atoms across a doped layer.

Continuing with the inverted doping structure, an GaAs bottom barrier150 may then be grown. The GaAs bottom barrier 150 may have a thicknessin the range of about 30 Å to about 100 Å, including all values andincrements therein. A Ge active channel layer 160 may then be grown onthe GaAs bottom barrier 150. The Ge active channel layer 160 may begrown to a thickness in the range of about 100 Å to about 500 Å,including all values and increments therein, at a temperature in therange of about 350° C. to about 500° C., including all values andincrements therein. The thickness of the GaAs bottom barrier 150 mayaffect charge carrier density in the Ge active channel layer 160. Arelatively thinner GaAs bottom barrier 150 may provide greater carrierdensity in the Ge active channel layer but may reduce carrier mobilitybecause of scattering between the carriers and dopant. A relativelythicker GaAs bottom barrier 150 may reduce carrier density but may notdecrease carrier mobility because the relatively thicker GaAs bottombarrier 150 may reduce scattering.

An AlAs top barrier 170 may then be grown on the Ge active channel layer160 at a temperature in the range of about 400° C. to about 600° C.,including all values and increments therein. The AlAs top barrier 170may be grown to a thickness in the range of about 100 Å to about 200 Å,including all values and increments therein. The Ge active channel layer160 may be a quantum well. A quantum well may be understood to be apotential well that may confine particles in one dimension and may,therefore, cause them to occupy a planar region. Finally, a GaAs contactlayer 180 may be grown to a thickness in the range of about 100 Å toabout 500 Å, including all values and increments therein, on the AlAstop barrier 170.

FIG. 1B depicts another representation of a layer structure of asemiconductor device 100′ in accordance with another exemplaryembodiment of the present disclosure. Layers 110, 120, 130, 140, 160 and180 and doped layer 145 may be the same as the layers with likedesignations depicted in FIG. 1A. In this embodiment, an AlAs bottombarrier 150′ may be grown prior to growth of the Ge active channel layer160. Similar to the embodiment depicted in FIG. 1A, following the growthof the Ge active channel layer, an AlAs top barrier 170 may be grown.The GaAs contact layer 180 may then be grown on the AlAs top barrier170.

Mismatched lattice constants between active device layers and adjacentlayers may result in defects (e.g., dislocations, stacking faults, twins(i.e., breaks in the periodic arrangement of atoms)) that may degradethe operation of a semiconductor device. FIG. 2 depicts band diagramsversus lattice parameters for a plurality of semiconductor materials.The Si (FIG. 2, 240) substrate 110 may have a lattice constant of about5.431 Å (Angstrom). The GaAs (FIG. 2, 220) layers 120, 130, 140, 150,180 may have a lattice constant of about 5.653 Å. The AlAs (FIG. 2, 230)layers 150′, 170 may have a lattice constant of about 5.660 Å. The Ge(FIG. 2, 210) active channel layer 160 may have a lattice constant ofabout 5.658 Å. The AlAs layers 150′, 170 and the Ge active channel layer160 may be considered to have relatively closely matched latticeconstants with a difference of about 0.04%. The GaAs layers 120, 130,140, 150, 180 and the Ge active channel layer 160 may be considered tohave relatively matched lattice constants with a difference of about0.09%. The Si substrate 110 and the Ge active channel layer 160 may beconsidered to have mismatched lattice constants with a difference ofabout 4%. GaAs layers 120, 130 and 140 and the GaAs and AlAs bottombarriers 150, 150′ may bridge the lattice mismatch between the Sisubstrate 110 and the Ge active channel layer 160. The lattices of theGaAs and AlAs bottom barriers 150, 150′ may sufficiently match thelattice of the Ge active channel layer 160 so that associated defects,e.g., dislocations, that may be present in the Ge active channel layer160 due to lattice mismatch may be minimized and/or may not propagate.

FIG. 3 depicts an exemplary schematic representation of GaAs 320 on a Sisubstrate 330. GaAs may be a polar material meaning GaAs may form bothcovalent and ionic bonds. Si may be a nonpolar material, i.e., Si mayform only covalent bonds. Growth of a nucleation layer between anonpolar substrate and a polar material may improve bonding between thepolar material and the substrate and may reduce antiphase domains.Antiphase domains may be bonds of Ga—Ga or As—As, for example, that mayincrease device leakage. FIG. 3 illustrates a single domain GaAs layer320 that may have been grown on a Si substrate 330. In other words, theGaAs layer 320 may not have antiphase domain defects.

FIG. 4 depicts an exemplary TEM image of an 0.8 μm thick GaAs layer 420grown on Si 430. GaAs and Si may not be relatively lattice matched andmay have a lattice mismatch of about 4%. As may be seen in FIG. 4,defects 440 may be present at the interface 410 between the Si layer 430and the GaAs layer 420. As may be further seen, for example, in FIG. 4,the defect density may decrease with increasing layer thickness, e.g.,region 440 as compared to region 450. The overall defect density may bedecreased through selection of growth conditions, e.g., thermal cycleannealing, growth rate, flux ratio of As (Arsenic) to Ga (Gallium), etc.

The magnitude of a valence band offset between the Ge active channellayer 160 and the GaAs and AlAs bottom barriers 150, 150′ may affecthole confinement within the Ge active channel layer 160. A greatervalence band offset may provide superior hole confinement than a lowervalence band offset. Superior hole confinement may then increase 2DHG(two dimensional hole gas) mobility. A two dimensional hole gas may bedefined as a gas of holes that may be free to move in two dimensions butmay be relatively tightly confined in a third dimension. For example, a2DHG may be present in a quantum well, e.g., 165 of FIGS. 1A and 1B. Asdiscussed above, increased hole mobility may provide faster switching.

FIGS. 5A and 5B depict, for example, valence band offsets 500, 500′ fortwo exemplary material interfaces. FIG. 5A illustrates a Ge activechannel layer 520 between a GaAs bottom layer 510 and an AlAs top layer530. As depicted in FIG. 5A, the GaAs bottom layer 510 may be grownfirst, followed by the Ge active channel layer 520 and then the AlAs toplayer 530. In this configuration, the GaAs-Ge interface 540 may have avalence band offset of about 0.42 eV for a Ge quantum well and about0.54 eV for an isolated interface. The Ge-AlAs interface 550 may have avalence band offset of about 0.65 eV for a Ge quantum well and about0.69 eV for an isolated interface. As a comparison, a Si_(1−x)Ge_(x)barrier layer with a Ge active channel layer may have a valence bandoffset in the range of about 0.2 to about 0.3 eV. A higher valence bandoffset may provide superior hole confinement. An AlAs—Ge (e.g., FIG. 1A,AlAs top barrier 170 and Ge active channel layer 160) interface mayprovide superior hole confinement as compared to a Si_(1−x)Ge_(x)—Geinterface.

FIG. 5B illustrates a Ge active channel layer 520 between an AlAs bottomlayer 510′ and a AlAs top layer 530. As depicted in FIG. 5B, the AlAsbottom layer 510′ may be grown first, followed by the Ge active channellayer 520 and then the AlAs top layer 530. In this configuration, theAlAs-Ge interface 550 may have a valence band offset of about 0.65 eVfor a Ge quantum well and about 0.69 eV for an isolated interface. TheGe-AlAs interface 540′ may have a valence band offset of about 0.91 eVfor a Ge quantum well and about 0.94 eV for an isolated interface. Asnoted above, a S_(1−x)Ge_(x) barrier layer with a Ge active channellayer may have a valence band offset in the range of about 0.2 to about0.3 eV. A higher valence band offset may provide superior holeconfinement. An AlAs—Ge (e.g., FIG. 1B, AlAs top barrier 170 and Geactive channel layer 160) interface may provide superior holeconfinement as compared to a Si_(1−x)Ge_(x)—Ge interface.

It may be appreciated that for a Ge quantum well configured, forexample, according to either FIG. 5A or FIG. 5B, the carriers (holes)may be confined inside the quantum well. In other words, such Ge quantumwell may be described as a type I quantum well. A type I quantum wellmay be defined as a quantum well having band offsets such that theconduction band edge of a barrier layer or layers is higher in energythan the conduction band edge of the quantum well layer or layers andthe valence band edge of the barrier layer or layers is lower in energythan the valance band edge of the quantum well layer or layers such thatthe wave functions for the lowest conduction sub-band and highestvalence sub-band may be localized in the same quantum well layer orlayers. Conversely, a Si_(1−x)Ge_(x) bottom layer with a Ge activechannel layer may provide a Type II quantum well where the wavefunctions for the lowest conduction sub-band and highest valencesub-band may be localized primarily in different quantum well layer orlayers.

The magnitudes of the band gaps of the GaAs and AlAs bottom barriers150, 150′ and the AlAs top barrier 170 may affect parallel conductionbetween the Ge active channel layer 160 and the buffer and/or GaAscontact layers. Parallel conduction may lead to a very smallI_(on)/I_(off) ratio in Ge quantum well based transistors. Further, theGe quantum well based transistors may require a higher drive voltage toturn them off for both long channel and short channel devices. Parallelconduction may also detrimentally affect (e.g., reduce) effective holemobility in an active device channel.

Referring to FIG. 2, GaAs 220 may have a band gap of about 1.424 eV andAlAs 230 may have a band gap of about 2.18 eV. Ge 210 may have a bandgap of about 0.67 eV and S±240 may have a band gap of about 1.1 eV. Acombination of Si and Ge (Si_(1−x)Ge_(x)) may then have a band gapbetween about 0.67 eV and about 1.1 eV, depending upon the value of theparameter x. It may be appreciated that both GaAs and AlAs may have bandgaps greater than the band gaps of S_(1−x)Ge and Si_(1−x)Ge_(x). Higherband gaps may correspond to materials having insulating properties whilesmaller band gaps may correspond to materials that act more likeconductors. Accordingly, layers of GaAs and AlAs, e.g., 150, 150′ and170, may provide superior isolation to the Ge active channel layer 160than layers of Si_(1−x)Ge_(x).

Various features, aspects, and embodiments have been described herein.The features, aspects, and embodiments are susceptible to combinationwith one another as well as to variation and modification, as will beunderstood by those having skill in the art. The present disclosureshould, therefore, be considered to encompass such combinations,variations, and modifications.

1. A semiconductor device, comprising: a Si substrate; a buffer layer onsaid Si substrate, said buffer layer comprising a GaAs nucleation layer,a first GaAs buffer layer, and a second GaAs buffer layer; a bottombarrier on said buffer layer, said bottom barrier having a band gapgreater than about 1.1 eV; a Ge active channel layer on said bottombarrier wherein a valence band offset between said bottom barrier andsaid Ge active channel layer is greater than about 0.3 eV; and an AlAstop barrier on said Ge active channel layer wherein said AlAs topbarrier has a band gap greater than about 1.1 eV.
 2. The semiconductordevice of claim 1, wherein: said bottom barrier is GaAs.
 3. Thesemiconductor device of claim 1, wherein: said bottom barrier is AlAs.4. The semiconductor device of claim 1, wherein: said Si substrate has a(100) offcut in the range of about 2 to about 8 degrees toward the [110]direction.
 5. The semiconductor device of claim 1, wherein: said GaAsnucleation layer has a thickness in the range of about 30 Å(Angstrom) toabout 500 Å, said first GaAs buffer layer has a thickness in the rangeof about 0.2 μm to about 1.0 μm, and said second GaAs buffer layer has athickness in the range of about 0.2 μm to about 5.0 μm.
 6. Thesemiconductor device of claim 1, further comprising: a first doped layeron said second GaAs buffer layer, wherein said bottom barrier is on saidfirst doped layer, wherein a dopant is Beryllium or Carbon.
 7. Thesemiconductor device of claim 1, further comprising: a second dopedlayer on said AlAs top barrier, wherein a dopant is Beryllium or Carbon.8. A semiconductor device comprising: a Si substrate having a (100)offcut toward the [110] direction; a buffer layer on said Si substrate,said buffer layer comprising a GaAs nucleation layer, a first GaAsbuffer layer, and a second GaAs buffer layer; a bottom barrier on saidbuffer layer wherein said bottom barrier has a band gap greater thanabout 1.1 eV; a Ge active channel layer on said bottom barrier wherein avalence band offset between said bottom barrier and said Ge activechannel layer is greater than about 0.3 eV; an AlAs top barrier on saidGe active channel layer wherein said AlAs top barrier has a band gapgreater than about 1.1 eV; and a GaAs contact layer on said AlAs topbarrier.
 9. The semiconductor device of claim 8, wherein: said offcut isin the range of about 2 to about 8 degrees.
 10. The semiconductor deviceof claim 8, wherein: said bottom barrier is GaAs.
 11. The semiconductordevice of claim 8, wherein: said bottom barrier is AlAs.
 12. Thesemiconductor device of claim 8, wherein: said GaAs nucleation layer hasa thickness in the range of about 30 Å to about 500 Å, said first GaAsbuffer layer has a thickness in the range of about 0.2 μm to about 1.0μm, and said second GaAs buffer layer has a thickness in the range ofabout 0.2 μm to about 5.0 μm.
 13. The semiconductor device of claim 8,further comprising: a first doped layer on said second GaAs bufferlayer, wherein said bottom barrier is on said first doped layer, whereina dopant is Beryllium or Carbon.
 14. The semiconductor device of claim8, further comprising: a second doped layer on said AlAs top barrier,wherein a dopant is Beryllium or Carbon.